The invention relates generally to a process to prepare drawn melt-spun polymeric fibers colored during the melt-spinning process. More particularly, the invention relates to a process to form polyamide or polyester based melt-colored fibers, containing novel polyester ionomer additive(s), that exhibit improved color and aesthetics over prior art melt-colored fibers.
Coloration of fibers has a long history, and the science of dyeing, initially of natural fibers such as flax, cotton and wool, has been under continuous development since Neolithic times. The appearance of man-made fibers, (e.g. cellulosics, acrylics, polyamides and polyesters), stimulated further developments in dyeing, and this method of coloring of fibers and articles made therefrom continues to be the most-practised technique for the production of colored fiber-based articles of manufacture.
In the case of fibers based on the more recent polymers such as polyamides and polyesters, which are spun from the melt, there exists an alternative method for coloration, i.e. addition of the colorant species into said melt and direct extrusion of colored fibers. While such a process may be carried out with dyes, it is more often carried out with pigments, (although the said process is popularly known in the industry as xe2x80x9csolution dyeingxe2x80x9d). The major difference between dyes and pigments is that, under prevailing processing conditions, pigments are virtually insoluble in polymers, whereas dyes are soluble, (see definitions in German Standards DIN 55943, 55944 and 55949).
As the technique of melt-pigmentation has been developed, it has been demonstrated that fibers made in this way can exhibit certain advantages over those made by post-spinning dyeing of fibers. Such advantages includexe2x80x94improvements to resistance to degradation and fading in sunlight; lower susceptibility to fading and/or yellowing by polluting gases in the atmosphere, such as ozone and nitrogen oxides; improved resistance to chemicals, either in dry-cleaning processes or due to accidental spillage; less leaching or fading of color during laundering or cleaning process involving water and detergents; no need for post-spinning dyeing, or the other processes involved in applying and fixing said dyes onto/into the fiber.
However, melt-pigmentation is also considered to have some disadvantages in terms of the color and appearance obtained in the final fiber. The fibers are known to those skilled in the art to exhibit degrees of lustre and low brightness which can render the said fibers unsuitable in certain applications.
The color change resulting from the addition of pigments to polymers is based on the wavelength-dependant absorption and scattering of light, the appearance and color of the final product being a combination of these two factors as described in the Kubelka-Munk theory. [Note that a description of this theory, and the general concepts of color and its measurement may be found in xe2x80x9cColour Physics for Industryxe2x80x9d, Roderick McDonald (Ed.), The Society of Dyers and Colourists, Bradford, UK, 2nd edition (1997)]. Dyes can only absorb light and not scatter it, since the physical prerequisite for scatteringxe2x80x94a certain minimum particle sizexe2x80x94does not exist in the case of dyes in molecular solution; these colors are therefor transparent. In so far as the transparency is attributable to the dye, complete absorption of the light will result in black shades, selected absorption in colored shades.
The optical effect of pigments may in the same way be based on light absorption. If, however, the refractive index of the pigment differs appreciably from that of the polymer (which is almost always the case) and, if a specific particle size range is present, scattering takes place; the initially transparent polymer becomes white and opaque, or if selective absorption takes place at the same time, colored and opaque.
No scattering occurs when the particle sizes are very small, and none or very little if they are very large. With all colored pigments that selectively absorb, the shade and strength of the final color is thus influenced by particle size. The transparency and thickness of the colored substrate may additionally affect the color strength. While some pigments are available in so-called transparent grades (e.g. red and yellow iron oxides) a complete color range across the spectrum is not readily available. Many such ultra-low particle size colorants are expensive, and difficult to maintain at high dispersion when compounded into a polymer matrix. It is also known that very low particle size additives in polymer melts can exhibit a profound effect on the Theological properties of said melt, resulting in formulations which are difficult to spin using standard equipment and techniques. Use of large particle size pigments is not a viable option either, as such additives will result in blocking of filtration systems and spinneret holes, and tend to lead to filament breaks in fiber production.
In any case, a large number of melt-pigmented fiber products are required to be opaque, and the problem lies in producing opaque colored fibers with levels of color brightness close to those of dyed products. Note that a fundamental difference between dyeing and pigmenting of fibers is that, while dyes are either colored or absorb all wavelengths (i.e. give black shades), pigmentation introduces an extra variable in that white pigments are readily available, whereas there is no such species as a xe2x80x9cwhite dyexe2x80x9d.
Another potential problem with particulate colorants in a polymeric melt-extruded product is the phenomenon of dichroism, or optical anisotropy. Pigment particles are not necessarily isotropic in shape, and may be needle-shaped, rod-shaped, or platelets. They may thus become oriented in a particular direction in processing. The apparent color then depends on the direction of observation. The origin of this phenomenon is to be found in the fact that certain pigments crystallise in crystal systems of low symmetry resulting in directionally dependant physical properties. As far as the coloristic properties are concerned, this signifies that the absorption and scattering constants differ in the various principle crystallographic axes, i.e. such crystals are optically anisotropic.
With regard to the final fiber (as opposed to the above comments on the pigments themselves) the appearance of a sample thereof can vary depending on the angle of illumination and/or observation. Fiber samples are normally prepared for color and appearance testing by carefully wrapping the fiber or yarn sample (under conditions of uniform tension and consistent positioning of the said fibers) around a flat xe2x80x9ccardxe2x80x9d, and assessing the color properties, and more importantly any differences between said properties and those of the desired standard sample or data, under standard conditions of illumination and observation. This may be carried out visually, but more usually instrumentally. Methods and apparatus for carrying out the analysis of color and appearance in this manner are well known to those skilled in the art.
During such examinations, there are two additional effects which might be observed if the sample is illuminated or observed at a number of different angles (for an example of multi-angle appearance testing of materials see U.S. Pat. No. 4,479,718, assigned to DuPont). The total amount of light reflected from the sample, per unit area, may change. The perceived color may also change. Unless such effects are specifically desired for particular aesthetic effects in a final article of manufacture, the appearance of either may result in the rejection of the fiber by the prospective customer. Eradication or reduction of such effects thus is important in obtaining first quality product.
There thus exists a need in the industry for a simple method to provide pigmented polymeric articles, especially melt-spun fibers, with improved color and appearance, whilst still using standard grades of pigment currently known to those skilled in the art to be suitable for this purpose, and equipment and techniques known to produce melt-pigmented fibers of the required properties for use in manufactured articles such as fabrics, textiles, carpets, threads, etc.
Vonk (U.S. Pat. No. 5,674,948) has claimed that end-capping of the amino end-groups of polyamides results in improvements to the brightness of compositions colored with organic pigments which are capable of themselves reacting with such amino end-groups. N-acetyl lactam is noted as a particularly preferred end-capper. This approach does not, however, fully solve the problem. The effect is limited to a particular set of organic pigments, and appears to be limited to use for colored polyamides only. The levels of additive end-capper are also quite high, in order to produce a sufficient reduction in amine end-group level to produce a noticeable effect. Finally, the use of this method requires a prior knowledge of the amine end-group level in the polyamide, (a non-trivial analytical task), and requires that the additive end-capper level be varied from polyamide to polyamide, (and potentially from batch to batch of the same polyamide), to account for the differing levels of amine end-groups therein.
The present inventors have discovered the surprising fact that addition of sulfonated polyester ionomer copolymers, at low levels, and irrespective of the type of colorant employed, results in the production of polyamide or polyester melt-pigmented fibers which exhibit improved color and appearance and reduced variation in color and appearance. The improvement in appearance between two samples can be measured as color strength. In order to fully achieve improved color and appearance, i.e. color strength, the fibers should be drawn.
The polymer used as the base, or matrix, polymer in the practise of this invention is selected from the set consisting of fiber-forming polyamides or polyesters.
Polyamides include those synthesised from lactams, alpha-omega amino acids, and pairs of diacids and diamines. Such polyamides include, but are not limited to, polycaprolactam [polyamide 6], polyundecanolactam [polyamide 11], polylauryllactam [polyamide 12], poly(hexamethylene adipamide) [polyamide 6,6], poly(hexamethylene sebacamide) [polyamide 6,10], poly(hexamethylene dodecanediamide) [polyamide 6,12], and copolymers and blends thereof. Preferred polyamides are polyamide 6 and polyamide 6,6.
Polyesters include those synthesised from one or more diacids and one or more glycols. Such polyesters include, but are not limited to, poly(ethylene terephthalate) [PET], poly(propylene terephthalate) [PPT], poly(butylene terephthalate) [PBT], poly(ethylene naphthalate) [PEN], polybutylene naphthanoate [PBN], polypropylene naphthanoate [PPN], polycyclohexane dimethanol terephthalate [PCT] and copolymers and blends thereof. Preferred polyesters are PET, PBT and PPT.
Colorants used in the practise of this invention may be selected from the categories of dyes, inorganic or organic pigments, or mixtures of these. Any number of different colorants may be used, in any proportions, although it will be understood by those skilled in the art that the total loading of colorants in the polymer matrix, and the number of different colorants used, will be kept to a minimum commensurate with obtaining the color required. Generally the level of colorants will range from 0.1 to 8.0% of the fiber weight.
Inorganic pigment types include, but are not limited to, elements, oxides, mixed oxides, sulfides, aluminates, sodium sulfo-silicates, sulfates and chromates. Non-limiting examples of these include: carbon blacks, zinc oxide, titanium dioxides, zinc sulfides, zinc ferrites, iron oxides, ultramarine blue, Pigment Brown 24, Pigment Red 101, and Pigment Yellow 119.
Organic pigment types include, but are not limited to, azos, disazos, quinacridones, perylenes, naphthalene tetracarboxylic acids, flavanthrones, isoindolinones, tetrachloroisoindolinones, anthraquinones, anthanthrones, dioxazines, phthalocyanines, and azo lakes. Non-limiting examples of these include Pigment Blue 60, Pigment Red 122, Pigment Red 149, Pigment Red 177, Pigment Red 179, Pigment Red 202, Pigment Violet 29, Pigment Blue 15, Pigment Green 7, and Pigment Yellow 150.
Colorants may be added to the matrix polymer in a variety of ways. These include direct addition of colorant(s) to the matrix polymer, addition of single-pigment dispersions (i.e. addition of each colorant as a separate concentrate in a carrier resin), and addition of multiple colorant dispersions (i.e. addition of a single concentrate of mixed colorants, providing the desired color on let down into the matrix polymer, in a carrier resin). Any of these addition methods may be carried out as a separate compounding step prior to melt-spinning, or may be carried out on the melt-spinning apparatus itself. In either case, the colorant(s) may be added at any stage of the process, for example at the extruder throat, at any addition port on the extruder barrel, at the melt pump, or at the spinneret chamber. More than one addition point may be utilised in the case of single-pigment dispersions.
The carrier resin used to produce pigment concentrates may be selected from a set consisting of low or high molecular weight polymers which have suitable compatibility with the matrix resin. One ordinarily skilled in the art of polymer compounding and blending will be familiar with which carrier polymers should be used to obtain suitable results in each matrix resin. Preferred carrier resins are PET, PBT, PPT, sulfonated polyester ionomer copolymers, polyamide 6, polyamide 11, polyamide 12, polyamide 6,6, polyamide 6,10, polyamide 6,12, and copolymers and blends thereof.
Polyester ionomers, in the context of the present invention, are defined as polyester polymers derived from the reaction residue of an aryl carboxylic acid sulfonate salt, an aromatic dicarboxylic acid, an aliphatic diol or any of their ester-forming derivatives. The said polyester ionomers comprise some monovalent and/or divalent sulfonate salt units represented by the formula 1A:
(Mn+O3S)d-A-(Cxe2x95x90O)pxe2x88x92
or formula 1B:
(Mn+O3S)d-A-(ORxe2x80x3OH)p
wherein p=1-3, d=1-3, and p+d=2-6, and A is an aryl group containing one or more aromatic rings: for example, benzene, naphthalene, anthracene, biphenyl, terphenyl, oxy diphenyl, sulfonyl, diphenyl or alkyl diphenyl, where the sulfonate substituent is directly attached to an aryl ring. These groups are incorporated into the polyester through carboxylic ester linkages. The aryl groups may contain one or more sulfonate substituents (d=1-3) and may have one or more carboxylic acid linkages (p=1-3). Groups with one sulfonate substituent (d=1) and two carboxylic linkages (p=2) are preferred. M is a metal, with valency n=1-5. Preferred metals are alkali metals or alkaline earth metals, where n=1 or 2. Zinc and tin are also preferred metals. Rxe2x80x3 is an alkyl spacer group: for example, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2OCH2CH2xe2x80x94, xe2x80x94CH(CH3)CH2xe2x80x94, xe2x80x94CH2CH2CH2xe2x80x94, xe2x80x94CH2CH2CH2CH2xe2x80x94.
Typical sulfonate substituents that may be incorporated into the metal sulfonate polyester copolymer may be derived from the following carboxylic acids or their ester-forming derivatives: sodium sulfoisophthalic acid, potassium sulfoterephthalic acid, sodium sulfonaphthalenedicarboxylic acid, calcium sulfoisophthalate, potassium 4,4xe2x80x2-di(carbomethoxy)biphenyl sulfonate, lithium 3,5-di(carbomethoxy)benzene sulfonate, sodium p-carboxymethoxybenzene sulfonate, dipotassium 5-carbomethoxy-1,3-disulfonate, sodio sulfonaphthalene-2,7-dicarboxylic acid, 4-lithio sulfophenyl-3,5-dicarboxybenzene sulfonate, 6-sodio sulfo-2-naphthyl-3,5-dicarbomethoxybenzene sulfonate and dimethyl-5-[4-(sodiosulfo)phenoxy] isophthalate. Other suitable sulfonate carboxylic acids and their ester-forming derivatives are described in U.S. Pat. Nos. 3,018,272 and 3,546,008 herein incorporated by reference. The most preferred sulfonate polyesters are derived from lithium or sodium 3,5-dicarbomethoxybenzene sulfonate.
Preferred ionomer polyester polymer comprises divalent ionomer units represented by the formula 2:
xe2x80x94(Cxe2x95x90O)-Ph(R)(SO3xe2x88x92M+)-(Cxe2x95x90O)xe2x80x94
wherein R is hydrogen, halogen, alkyl or aryl, and M is a metal.
The most preferred polyester ionomer has the formula 3:
where the ionomer units, x, are from 0.1-50 mole percent of the polymer with 5 to 13 mole percent being preferred and 8 to 12 mole percent being especially preferred. Most preferably R is hydrogen. It is also preferred that the polyester sulfonate resin be water insoluble. In general water insoluble resins will be of high molecular weight (IV greater than or equal to 0.2dl/g in 60/40 phenol/tetrachloroethane solution) and have less than 15 mole percent sulfonate units in the polyester chain. Sulfonate copolymer polyester resins with IVxe2x89xa70.3dl/g and with 8-12 mole percent sulfonate units are preferred.
Typical glycol or diol reactants, R1, include straight chain, branched or cycloaliphatic alkane diols and may contain from 2 to 12 carbon atoms. Examples of such diols include, but are not limited to, ethylene glycol; propylene glycol, i.e. 1,2- and 1,3-propanediol; butane diol, i.e. 1,3- and 1,4-butanediol; diethylene glycol; 2,2-dimethyl-1,3-propanediol; 2-ethyl-2-methyl-1,3-propanediol; 1,3-pentanediol; 1,5-pentanediol, dipropylene glycol; 2-methyl-1,5-pentanediol; 1,6-hexanediol, dimethanol decalin; dimethanol bicyclooctane; 1,4-cyclohexane dimethanol and particularly its cis- and trans-isomers; triethylene glycol, 1,10-decanediol; and mixtures of any of the foregoing. A preferred cycloaliphatic diol is 1,4-cyclohexane dimethanol or its chemical equivalent. When cycloaliphatic diols are used as the diol component, a mixture of cis- and trans-isomers may be used, it is preferred to have a trans-isomer content of 70% or more. Chemical equivalents to the diols include esters, such as dialkyl esters, diaryl esters and the like.
Examples of aromatic dicarboxylic acid reactants, as represented by the decarboxylated residue A1, are isophthalic acid or terephthalic acid, 1,2-di(p-carboxyphenyl)ethane, 4,4xe2x80x2-dicarboxydiphenyl ether-4,4xe2x80x2-bisbenzoic acid and mixtures thereof. All of these acids contain at least one aromatic nucleus. Acids containing fused rings can also be present, such as in 1,4-, 1,5- or 2,6-naphthalenedicarboxylic acids. The preferred dicarboxylic acids are terephthalic acid, isophthalic acid or mixtures thereof.
The most preferred ionomer copolyesters are poly(ethylene terephthalate) [PET], poly(propylene terephthalate) [PPT] or poly(1,4-butylene terephthalate) [PBT] ionomers. It is most preferred that the polyester metal sulfonate salt copolymers be insoluble in water.
Also contemplated herein are the above ionomers with minor amounts, e.g. from about 0.5 to about 15 percent by weight, of units derived from aliphatic acids and/or aliphatic polyols to form copolyesters. The aliphatic polyols include glycols, such as poly(ethylene glycol) or poly(butylene glycol). Such copolyesters can be made following the teachings of, for example, U.S. Pat. Nos. 2,465,319 and 3,047,539.
The polyester ionomers described above may be incorporated into the matrix polymer either during a melt-compounding step prior to fiber spinning, or during the fiber spinning process itself. These may be added in a separate process to the addition of the pigments, or may be added along with the said pigments. If the latter, it may be that the polyester ionomers are used as carrier resins for the said colorants, although this is not necessary for the practise of the present invention.
The amount of polyester ionomer(s) used in the practise of the present invention will vary depending on the types and amounts of colorants used to obtain any particular color. It has been found, in the course of extensive experimentation, that addition of said polyester ionomers to the melt formulation for spinning into fiber which produces a final sulfur level of between about 200 and about 5000 ppm gives the best results. The sulfonate polyester copolymers are to be used at 0.5 to 30% by weight of the fiber.
Besides the matrix polymers, colorants and polyester ionomers described above, the formulations used in the practise of the present invention may contain other components. These may include, but are not limited toxe2x80x94antioxidants, UV stabilisers, antiozonants, soilproofing agents, stainproofing agents, antistatic additives, antimicrobial agents, lubricants, melt viscosity enhancers, flame retardants and processing aids.
A formulation used in the practise of the present invention comprising:
1) A matrix polymer, selected from the set of fiber-forming polyamides and fiber-forming polyesters, as this is defined above;
2) A colorant system comprising of one or more colorants selected from the sets of inorganic and/or organic colorants as these are defined above, said colorant system optionally including one or more carrier resins for said pigments;
3) One or more sulfonated polyester copolymers, as these are defined above.
As stated previously, any or all of the above noted ingredients may be combined in a number of ways, either in a separate compounding step prior to actual melt-spinning of the fibers, or during the fiber spinning process itself. Both the separate compounding process and the fiber spinning process may be carried out using techniques and equipment well known to those ordinarily skilled in the arts of polymer compounding and fiber melt-spinning.
In order to achieve the full improvement in color strength and appearance it is necessary to draw the fibers formed from the sulfonated polyester blends. The optimum draw ratio will vary with the exact nature of the fiber matrix, colorant type and loading, sulfonate polyester type and concentration, fiber diameter and cross section shape. The draw ratio is defined as the ratio of the final length to the original length per unit weight of the yarn resulting from the drawing process. The optimum draw ratio for a given system can readily be determined by comparing the color strength of as spun fiber with that of the same fiber drawn under different conditions. In general a draw ratio from 1.05 to 7.00 may be used, with a draw ratio of 1.10 to 6.00 being more preferred. Fiber drawing may be achieved by any standard methods known in the art. The fiber may be drawn between two godet rolls, or pairs of rolls, or over a draw pin or pins, or a mixture of the two. The drawing may be done in single or multiple stages. The fiber is usually heated to a temperature above the glass transition temperature prior to, or during, drawing to minimise fiber breakage, although heating is not a requirement for the invention. The heating may be carried out via heating of godet rolls, plates, slits, pins or other means such as use of a heated chamber; hot gas such as steam, or hot liquid, such as water, may also be used.
The fibers produced from the practise of the present invention may be of a range of deniers per filament (dpf) depending on the ultimate use to which such fibers may be putxe2x80x94low dpf for textile use, higher dpf for use in carpets. The cross-sectional shape of the fibers may also be any of a wide range of possible shapes, including round, delta, trilobal, tetralobal, grooved, or irregular. These product fibers may be subjected to any of the known downstream processes normally carried out on melt-spun fibers, including crimping, bulking, twisting etc., to produce yarns suitable for incorporation into a variety of articles of manufacture, such as apparel, threads, textiles, upholstery and carpets.